Pickling treatment



Patented Aug. 14, 1951 UNITED STATES PATENT OFFICE 2,564,549 PICKLINGTREATMENT Albert R. Stargardter, Baltimore, Md.

No Drawing. Application July 2, 1945, Serial N0. 602,936

1 Claim. 1

In the manufacture of stainless steel into wrought form the materialmust be rolled at high temperatures, followed by annealing, ornormalizing, also at high temperatures, with the attendant formation ofa surface film of oxide scale, which must be completely removed in orderto render the stainless steel articles corrosion resistant and fit forgeneral use.

My invention relates to the removal of scale from ferrous metals,particularly alloys of the type commonly referred to as stainless steel,which contain iron, nickel, and chromium as essential ingredients, by anew type of pickling solution which retains its effectiveness duringuse, since it can be constantly controlled and regenerated and whichalso has a substantially negligible solvent action on the stainlesssteel base metal.

To the best of my knowledge, current practice in pickling the stainlesssteels includes two operations, the first known as the pro-pickling,followed by the second, which includes immersion in a final picklingbath. The pre-pickle usually consists of hot dilute sulphuric acid, orsulphuric acid plus sodium chloride, or sulphuric acid plus ferricsulphate. The function of this first step is to secure preliminary scaleattack. The actual pickling, or scale removal, is accomplished in thefinal pickling bath, which usually consists of a hot solution of nitric,plus hydrofluoric acids.

This final pickle leaves a clean, white surface, but as currentlypracticed is accompanied by two undesirable features. First, thesolution is highly corrosive to the stainless steel base metal, so thatits use is accompanied by metal loss, which in the case of thin materialsuch as sheets will constantly run in excess of one per cent. Secondly,the use of this solution results in the evolution of noxious, highlytoxic fumes of hydrofluoric acid and nitrogen oxide vapors.

The use of ferric sulphate, plus hydrofluoric acid, has been tried forpickling stainless steel, but although its action is far superior to thenitric-hydrofluoric bath, it is not practicable, since up to the presenttime, no means have been offered for regenerating this bath, exceptthrough addition of increments of ferric sulphate, so that in a space oftime far too uneconomical for practical purposes, the bath becomessaturated with iron salts. It is for this reason that this process couldnot be put into commercial practice.

My invention teaches the use of a soluble oxidizing agent as aregenerative means for control of the ferric sulphate-hydrofluoric acidpickling bath for stainless steel pickling. I have found that anoxidizing agent to be of a practical value must be:

1. Easily soluble in the pickling bath,

2. And capable of oxidizing ferrous iron to the ferric condition in hotacid solution, as well as 3. Form reaction products which are highlysoluble in the pickling bath, and

4. Form reaction products which are not deleterious to the functionalperformance of the pickling bath.

In order to successfully utilize ferric sulphatehydrofiuoric baths forthe pickling of stainless steel and which are highly desirable for thispurpose due to the absence of noxious fumes and saving of valuablestainless steel due to absence of pickling loss, it is essential toaccomplish their regeneration by the constant re-oxidation of theferrous sulphate. The latter is constantly being formed as adecomposition product of the original ferric sulphate as a result of thepickling action of said ferric sulphate. This regeneration must usuallybe accompanied by adequate additions of hydrofluoric acid.

I have discovered that chlorates, perchlorates,

" persulphates and chlorites of the alkaline earth and alkali metals andammonia, in general, offer adequate means for the re-oxidation offerrous sulphate, although other oxidizing agents having the aforesaidcharacteristics may be used. The most efiicient are the alkali metalcompounds notably potassium chlorate. Thus, I have found that a picklingbath for stainless steel composed of effective amounts of ferricsulphate. and hydrofluoric acid to accomplish the pickling action, plusthe addition of a suitable oxidizing agent, such as one of thosementioned above, more particularly potassium chlorate, serves to removescale from stainless steel surfaces in a manner superior to that of anyother form of pickling bath in use at the present time.

The following description of the operation of this bath will serve as anexample of an embodiment of my invention.

A 35,000 gallon bath was prepared consisting By weight Ferric sulphate16.0 Hydrofluoric acid 3.0 Potassium chlorate 0.3

Balance-water and was heated to and maintained at -170 F. Stainlesssteel sheets of the approximate composition ill-chrome, 8-nirrkel,balance-iron were pickled in the bath after a pre-pickle in a bathconsisting of sulphuric acid. 4%, plus ferric sulphate, 4%.Ansheswereimmeisea in the" pickling bath for tenminutesthlbat stantlystirred-by air agitation.

Per cent Ferrous s p 3.00 Ferric sulphate 12.1 Hydrofluoric acid 2.8

At this juncture, 20% of hydrofluoric acid was added, and the picklingwas continued. However, after 100 additional sheets of the same lot werepickled it became apparent that the bath needed Per cent Ferrous sulpha5.40 Ferric sulpha 9.40 Hydrofluoric acid 2.91

At this point .3% (100 lbs.) potassium chlorate and .2% hydrofluoricacid were added, and the pickling was continued on a 24 hour basis, thesheets being pickled in racks of 20 sheets at a time, and, as a constantpractice, .3% of potassium chlorate, plus .3% of hydrofluoric acid wereadded after every 35 lifts of 20 sheets each, i. e. after each lot of700 sheets with a total surface area of 42,000 sq. ft. were pickled.

In this manner, a total of 14,000 sheets, averaging 3' x 10 in size. andweighing approximately 500,000 lbs. were pickled in 12 consecutiveworking days, or 288 hours.

The control analyses taken each lot of 700 sheets averaged as follows: R

Per cent Ferrous sulphate 3.00 Ferric sulph 12.15 2.75

Hydrofiuoric acid and each time at this stage, rectifying additions of.3% potassium chlorate, plus ,.3% hydrofluoric acid were added, andafter each one of these additions the bath was analyzed and at thisstage the average analysis of the bath was:

Ferrous sulphate None Ferric sulphate per cent 16.5 Hydrofiuoric acid do3.00

Per cent Ferric sulphate 16.6 Ferrous sulphate 6.8 Hydrofiuoric acid2.65 Potassium chloride 4.4

Thus, the same bath was maintained with the picklin constituents inefiective amounts to carry out the pickling function and operatedconstantly for two weeks by means of the simple control consisting ofregular additions of small increments of potassium chlorate andhydrofluoric acid.

a After the first addition of 16% FeflSOOa. m' morewas added, it beingconstantly regenerated by the X6103 additions.

My control system resolved itself into the simple procedure of theaddition of .3% potassium chlorate and a .3% hydrofluoric acid for every42,000 sq. ft. of surface pickled.

- 7 There may be instances where addition of hydrofluoric acid is notalways necessary, but usually it is required along with the periodicaddition of the oxidizing agent. i

The present material cost for pickling by the nitric-hydrofluoric acidsystem is about $.0019 per pound of steel pickled, and my processrepresents a saving of about 20% in pickling materials. However,additional advantages were noted incident to the operation of my bath,such as:

1. Elimination of the nitrogen oxide and hydrofluoric fumes.

2. Virtual elimination of steel loss due to solution of the underlyingmetal into the bath.

By actual tests in picking 21 gauge stainless steel sheets of thel8-chrome, 8-nickel variety in the conventional nitric-hydrofluoricbath, a metal loss of 1.25% was encountered, while in tests run in thesame manner in my bath, as described above, the actual metal loss wasabout .l0%, this exceptionally low loss being attributable to thevirtual absence of ferrous sulphate under operating conditions. Thus, asaving of 23 pounds of stainless steel per ton is effected through theuse of my invention.

(Average value: 23 $.30=$6.90 per ton.)

It is generally agreed that the chemical reactions bringing aboutstainless steel pickling by means of ferric sulphate and hydrofluoricacid are not accurately known. However, it was found that due to therapid build-u of ferrous sulphate as a reaction product of this picklingin the absence of an oxidizin agent as described herein, baths of ferricsulphate, plus hydrofluoric acid became vitiated and inoperative aftersix or eight hours service .under conditions identical with thosemaintained in the 288 hour run described above, except by addition offurther Fez(S04)z as an attempt to regenerate. This latter expedient isimpracticable because a limiting amount of added Fez(SOi) 3 (7.00%) issoon reached.

The reaction in the pickling process of my mvention may be described asfollows:

in which it is most readily seen that the damaging reaction product ofthe pickling, viz. FeSO4 is regenerated by the addition of KClOa backand converted to its original form: Fe2(SO4)3.

The optimum ratio of ferric sulphate to hydrofluoric acid in thepractice of my invention is from four to six. Potassium chlorate is thepreferred re-energizing or oxidizing agent, and the proper amount of itsaddition to the bath when pickling stainless steel of the l8-chrome,8-nickel variety is approximately from two to three pounds of potassiumchlorate per 1,000 sq. ft. of stainless steel surface pickled. 7

Effective amounts of ferric sulphate (Fez(SO4) a) and hydrofluoric acid(HF) to satisfactorily ac complish the pickling function, of course, mayvary dependent on the composition of the metal or alloy being treatedand the condition and amount of the scale.

Generally, an aqueous bath containing about 10% to 20% of ferricsulphate and about 2.0%

to 5.0% of hydrofluoric acid will be effective to suitably pickle themetal and from about a trace to 2.0% of oxidizing agent (the remainderbeing water) will be effective to maintain the ferric sulphatecontinuously in the effective pickling range and the ferrous sulphatepresent below an amount which would cause -appreciable corrosion of themetal base surface.

I claim:

The process of pickling ferrous metal in an aqueous pickling bathconsisting essentially of ferric sulfate and hydrofluoric acidcomprising subjecting the ferrous metal to an aqueous pickling bathconsisting essentially of from about 10 to 20% ferric sulfate and fromabout 2 to 5% hydrofluoric acid, and an oxidizing agent selected fromthe group consisting of chlorates, perchlorates, persulfates andchlorites of the alkaline earth and alkali metals and ammonia in amountof from a trace to about 2.0%, the bath having a temperature of about160 to 170 F.,

said oxidizing agent continuously oxidizing ferrous sulfate as formed inthe bath in the pickling operation to ferric sulfate, and introducingadditional oxidizing agent when the amount of ferrous sulfate present isactive to appreciably corrode the metal, whereby the ferric sulfate isalways maintained in effective amount of about 10 to 20%.

ALBERT R. STARGARDTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,269,443 Hoffman -1 June 11,1918 2,211,400 Wood Aug. 13, 1940 2,337,062 Page Dec. 21, 1943 FOREIGNPATENTS Number Country Date 525,011 Great Britain Aug. 20, 1940

